Ti3C2Tx MXene Polymer Composites for Anticorrosion: An Overview and Perspective

As the most studied two-dimensional (2D) material from the MXene family, Ti3C2Tx has constantly gained interest from academia and industry. Ti3C2Tx MXene has the highest electrical conductivity (up to 24,000 S cm–1) and one of the highest stiffness values with a Young’s modulus of ∼ 334 GPa among water-dispersible conductive 2D materials. The negative surface charge of MXene helps to disperse it well in aqueous and other polar solvents. This solubility across a wide range of solvents, excellent interface interaction, tunable surface functionality, and stability with other organic/polymeric materials combined with the layered structure of Ti3C2Tx MXene make it a promising material for anticorrosion coatings. While there are many reviews on Ti3C2Tx MXene polymer composites for catalysis, flexible electronics, and energy storage, to our knowledge, no review has been published yet on MXenes’ anticorrosion applications. In this brief report, we summarize the current progress and the development of Ti3C2Tx polymer composites for anticorrosion. We also provide an outlook and discussion on possible ways to improve the exploitation of Ti3C2Tx polymer composites as anticorrosive materials. Finally, we provide a perspective beyond Ti3C2Tx MXene composition for the development of future anticorrosion coatings.


INTRODUCTION
Ti 3 C 2 T x , the most studied two-dimensional (2D) material in the MXene family, has gained great attention since its first synthesis in 2011. 1 The chemical formula of MXene indicates the number of atomic layers of the elements present in a sandwich-like layered morphology. For example, Ti 3 C 2 T x consists of three layers of Ti atoms and two layers of C atoms arranged in layers of Ti−C−Ti−C−Ti. The T x component in the formula represents the surface terminations (typically −OH, −F, −O, −Cl) existing on the outer planes of Ti as an outcome of the synthesis method. 1−3 Thus, it is easily well-dispersed in water or other solvents, with the highest electrical conductivity (up to 24,000 S cm −1 ) and Young's modulus (∼334 GPa) among all solution-processed 2D materials. 4−6 In addition, the top-down synthesis method via wet chemical selective etching from its precursor, the Ti 3 AlC 2 MAX phase, 7,8 makes it quite scalable for industrial synthesis. Owing to these superior properties and its feasibility for solution processability, scalability, and surface functionality, various applications of Ti 3 C 2 T x such as in catalysis, 9,10 electromagnetic interference shielding, 11 energy-storage applications, 12 flexible electronics, and biosensors 13 have been reported.
Corrosion is a tendency of a metal to convert to its oxide form. It has a significant environmental and economic impact on society. Unlike graphene, Ti 3 C 2 T x -based coatings for anticorrosion are not widely explored. For instance, Ti 3 C 2 T x MXene was projected as a robust current collector for water desalination applications 14 and lithium-ion batteries. 15 Chloride anions present in saline water could corrode the current collectors beyond threshold potentials, impacting the efficiencies. The use of Ti 3 C 2 T x MXene as a current collector is due to its high specific surface area, suitable pore structure, high redox activity, high electrical conductivity, and stability in aqueous electrolytes. These properties enable Ti 3 C 2 T x MXene electrode operation at a high salt adsorption capacity within a large voltage window of electrochemical stability, exhibiting high reversibility without corrosion. 14 However, despite these advantages, no works on grafting polymeric materials on Ti 3 C 2 T x for anticorrosion protection are reported to the best of our knowledge. For anticorrosive coating applications, some reports have investigated Ti 3 C 2 T x -based polymer composites by either noncovalent or covalent functionalization 16,17 with noncovalent functionalization, employing physical mixing of Ti 3 C 2 T x with polymeric materials. In the first part of this review, we discuss recent research on Ti 3 C 2 T x −polymer composites for anticorrosion. Next, we present how to improve Ti 3 C 2 T x MXene integration into polymer matrices to further enhance the anticorrosion properties of these materials. Finally, we provide a perspective beyond Ti 3 C 2 T x , which can be useful for the future development of anticorrosion coatings.

TI 3 C 2 T X MXENE POLYMER COMPOSITES FOR ANTICORROSION
Ti 3 C 2 T x MXene polymer composites prepared via surface functionalization will be discussed in detail in the subsequent sections. The discussion is organized as follows: (a) first, the efficacy of pristine MXenes/polymer matrix composites, followed by (b) MXene coatings derived via surface functionalization methods.

Pristine Ti 3 C 2 T x .
To exploit the anticorrosion properties of pristine Ti 3 C 2 T x nanosheets, single-to few-layer Ti 3 C 2 T x nanosheets were physically mixed via magnetic stirring in a waterborne epoxy coating (WEC) 18,19 or waterborne polyurethane (WPU). 20 The anticorrosion properties of pristine Ti 3 C 2 T x were first reported by Yan et al., 18 where they incorporated Ti 3 C 2 T x nanosheets in epoxy resin with an amine curing agent. Ti 3 C 2 T x exhibited stable dispersions in the epoxy matrix due to its hydrophilic nature, which is vital to create a perfect physical barrier for anticorrosion. Figure 1a shows the Tafel plots of the uncoated Q345 sample, pure epoxy, and Ti 3 C 2 T x /epoxy composites with different ratios of Ti 3 C 2 T x (0.5, 1, and 2 wt % Ti 3 C 2 T x /epoxy). After immersion in 3.5% NaCl solution for 96 h, the MXene showed enhanced corrosion protection on the steel substrates compared to pure epoxy coatings. The improvement in anticorrosion properties was attributed to the presence of MXene flakes as thin film barriers for the diffusion of electrolyte and providing corrosion protection to the substrate (Figure 1b). 18 Moreover, the 1 wt % Ti 3 C 2 T x -coated sample showed the highest protection, indicated by the shift of the potential (E corr ) to the most positive value and the lowest corrosion current (I corr ) as shown in Figure 1a. Table 1 shows the anticorrosion   parameters of each sample, as derived from the Tafel plot in Figure 1a. Figure 1c shows the optical photographs of pure epoxy and Ti 3 C 2 T x −epoxy coatings before (0 days) and after 15 days of exposure time in a salt spray chamber. In agreement with Figure 1a, 1.0 wt % Ti 3 C 2 T x −epoxy shows the highest corrosion protection, outperforming 2.0 wt % Ti 3 C 2 T x − epoxy. These results indicate that adding more Ti 3 C 2 T x does not necessarily result in higher protection. Rather, the optimized value for the mixing of Ti 3 C 2 T x and the epoxy where the best dispersibility can be obtained is more structurally important. The higher content of Ti 3 C 2 T x leads to the stacking of flakes, creating higher volume densities and resulting in phase aggregation and separation between Ti 3 C 2 and the epoxy matrix, further impeding the effectiveness of Ti 3 C 2 T x 2D flakes as a physical barrier for anticorrosion. These results corroborate the work on graphene-based coatings, where the optimized ratio of graphene and the polymer matrix is found to be important to achieve the highest degree of protection against corrosion. 21 Another aspect that reflects the anticorrosion properties is the impedance modulus at the lowest frequencies, |z| f = 0.01 Hz, where a higher impedance modulus results in better corrosion protection. In the same study, the authors observed that for the first 2 h, the impedance moduli were 9.42 × 10 7 , 3.55 × 10 8 , 6.23 × 10 8 , and 3.95 × 10 8 Ω cm 2 for pure epoxy, 0.5, 1, and 2 wt % Ti 3 C 2 T x , respectively. However, after 96 h immersion in saline environments (3.5% NaCl), the impedance moduli significantly decreased to 2.27 × 10 6 , 7.6 × 10 6 , 2.96 × 10 7 , and 6.11 × 10 6 Ω cm 2 , respectively. The decrease of the impedance modulus at longer exposures to saline environments is attributed to the instability of Ti 3 C 2 T x due to oxidation and hydrolysis. 22−24 It is known that despite its superior intrinsic properties, Ti 3 C 2 T x flakes are prone to hydrolysis and oxidation in hydrated environments and potential transformation to TiO x and TiO 2 . 17,23−25 The degradation is usually initiated at the defect and edge sites of MXenes and is a multistep process where the defect sites undergo systematic hydrolysis with the formation of TiO 2 as the final degraded product. 26 This chemical degradation may impede the anticorrosion behavior of MXene in the long term, therefore requiring strategies to mitigate oxidation.
The oxidation of MXenes due to the reaction with dissolved oxygen can be limited by storing them in solutions saturated with inert gas. 24 Another way to slow the oxidation rates of MXenes is by freezing at ultralow temperatures (−20 or −80°C ), extending the shelf life to two years. 27 Stability of MXene can be effectively improved by hydrogen annealing but causes loss of dispersibility in solvents and surface reactivity. 28 When excess aluminum was used in the synthesis mixture of Ti 3 AlC 2 resulting Ti 3 C 2 T x MXene had lower number of Ti and C vacancies and exhibited a better oxidation resistance. 29 A longterm storage of MXenes in an aqueous solution utilizing hydration chemistry with nontoxic inorganic salts inhibits the attack of MXene by free water and oxygen molecules. 30−32 As a result, oxidation can be largely inhibited, prolonging the shelf life to up to 400 days with negligible loss of surface chemistry. Other methods such as surface functionalization 33,34 with organic ligands can maintain MXene chemical stability for long-term applications such as anticorrosion additives.
2.2. Surface-Functionalized Ti 3 C 2 T x MXene. Aminopropyl triethoxysilane (APTES) is the most used silane for surface functionalization. The primary amino functional groups in APTES offer several possibilities for postfunctionalization from bioconjugation to nanoparticle impregnation. Ji et al. 35 reported that functionalized Ti 3 C 2 T x with APTES demonstrates improved stability against oxidation with adjustable hydrophilicity in comparison to pristine Ti 3 C 2 T x MXene. Yan et al. 36 were the first to investigate the anticorrosion properties of amino-functionalized Ti 3 C 2 T x MXene. In their work, APTES was first attached to Ti 3 C 2 T x via a simple wet deposition, using the abundant hydroxyl functional groups on Ti 3 C 2 T x in the colloidal solution form. Figure 2 shows the schematic illustration of the preparation of Ti 3 C 2 T x MXene and its surface functionalization with APTES. The aminofunctionalized Ti 3 C 2 T x exhibited higher mechanical properties and showed better dispersibility in water, in comparison to pristine Ti 3 C 2 T x . 36 Note that the synthesis of Ti 3 C 2 T x MXene has been reported in numerous studies and reviews elsewhere 3,37−40 and is not in the scope of this review.
Composites with 0.25 and 0.5 wt % of pristine Ti 3 C 2 T x (0.25 l-M and 0.5 l-M, respectively) and 0.25 and 0.5 wt % APTES-functionalized Ti 3 C 2 T x (0.25 and 0.5 f-M, respectively) in the waterborne epoxy polymer were prepared by physical mixing. The addition of pristine Ti 3 C 2 T x and APTESfunctionalized Ti 3 C 2 T x has enhanced the anticorrosion properties of the epoxy coatings ( Figure 3a) where MXene 2D flakes acted as a physical barrier to the corrosion agents (Figure 3b). The corrosion resistance was 2.34 × 10 8 Ω cm 2 for pure epoxy. The functionalized Ti 3 C 2 T x exhibits higher corrosion resistance in comparison to pristine Ti 3 C 2 T x . The 0.5 wt % APTESfunctionalized Ti 3 C 2 T x /epoxy coatings (f-M 0.5% ) demonstrated the best anticorrosion protection, as indicated by the most positive value in the Tafel plot ( Figure 3a and Table 2). f-M 0.5% showed the highest corrosion protection of 3.09 × 10 9 Ω cm 2 . However, after 4 weeks of immersion in 3.5% NaCl, a significant decrease in corrosion resistance was observed for all samples. The pure epoxy exhibited the highest degradation to 3.45 × 10 5 Ω cm 2 , whereas the 0.5 wt % APTES-functionalized Ti 3 C 2 T x /epoxy coatings exhibited the lowest degradation (from 3.09 × 10 9 to 1.02 × 10 7 Ω cm 2 ). The study shows the importance of the ligand functionalization of Ti 3 C 2 T x in improving its dispersibility as well as maintaining its chemical stability while slowing down degradation due to oxidation in the polymer matrix. 36 Similarly, Zhang et al., 41 reported surface functionalization of [3-(2-aminoethyl) aminopropyl] trimethoxysilane (AEAPTES) on Ti 3 C 2 T x MXene. The AEAPTES-functionalized Ti 3 C 2 T x (named Ti 3 C 2 @Si) was then mixed with waterborne polyurethane (WPU) with Ti 3 C 2 T x ratios of 0.05, 0.1, and 0.15 wt %. Figure 4 shows the Tafel plots for the uncoated steel substrate, pristine Ti 3 C 2 T x /WPU, and WPU and Ti 3 C 2 @Si. Ti 3 C 2 @Si exhibited a positive shift in the Tafel plot (Figure 4c). The 0.1 wt % Ti 3 C 2 T x sample showed the lowest current density, indicating the best anticorrosion performance, outperforming 0.15, 0.05 wt %, WPU, Ti 3 C 2 T x /WPU, and bare Q235 steel substrate.
The parameters extracted from the Tafel plots (Table 3) confirm better corrosion protection of the functionalized-Ti 3 C 2 T x coated samples in comparison to pristine Ti 3 C 2 T x MXene. The results indicated that pristine Ti 3 C 2 T x MXene in WPU was less effective in anticorrosion compared with pure WPU ( Table 3). The poor performance of pristine Ti 3 C 2 T x MXene in WPU may be attributed to the intermittent and noncontinuous interfacial adhesion between pristine Ti 3 C 2 T x and WPU. This may create spaces or micropores that facilitate the corrosive ions to permeate and subsequently propagate the degradation beneath the coating. All functionalized Ti 3 C 2 T x MXene−WPU coatings showed higher corrosion protection with 0.1 wt % exhibiting the lowest current density of 2.67 × 10 −9 A cm −2 and the highest contact resistance of 3.05 × 10 6 Ω·cm 2 . Impressively, after 42 days of immersion in 3.5% NaCl, no degradation was observed. This finding corroborates with the work of Yan et al., 36 where the surface functionalization of Ti 3 C 2 T x with APTES was found to enhance the chemical enhance and stabilize the corrosion resistance. Figure 5 shows the mechanism of the corrosion protection of pure WPU, Ti 3 C 2 T x /WPU, and functionalized Ti 3 C 2 @Si/ WPU. Steels coated with pure WPU corrode easily, probably due to the defects and micron-sized holes through which the  ions can permeate with relative ease. Contrary to expectations, Ti 3 C 2 /WPU coatings were less efficient than pristine WPU. This may be due to abundant oxygen functional groups on Ti 3 C 2 T x surfaces, attracting water molecules and other corrosive media, acting as initiator sites for oxidation. The introduction of amino functional groups on Ti 3 C 2 T x facilitates intercalation with isocyanate groups in WPU, giving a strong and compact structure and leading to stable dispersions of Ti 3 C 2 @Si in WPU. Therefore, a network of an effective barrier was formed by good compatibility and dispersibility while creating complex diffusion paths, thereby slowing the diffusion rates. Importantly, the functionalized Ti 3 C 2 @Si increased the hydrophobicity of WPU, which decreased the absorption of water and enhanced the corrosion performance of Ti 3 C 2 /WPU composite coatings. These findings are similar to those of graphene-based materials, 21,42,43 where covalently functionalized graphene demonstrated better corrosion protection compared to pristine graphene.

Ti 3 C 2 T x /PANI Composites.
Conductive polymers (CPs) such as polythiophene (PT), 44 polypyrrole (PPy), 45,46 and polyaniline (PANI) 47−49 exhibit anticorrosion properties. Among these, PANI has attracted more attention for anticorrosion owing to its ease of synthesis, thermal stability, and reversible acid/base doping/dedoping. 50 Cai et al. 51 combined PANI and the 2D Ti 3 C 2 T x to enhance the anticorrosion properties of waterborne epoxy (WEP) resins. Ti 3 C 2 T x /PANI composites were prepared via in situ polymerization (Figure 6a,b) in Ti 3 C 2 T x /PANI mass ratios of 1:1, 1:2, and 1:4, followed by sandwiching the Ti 3 C 2 T x /PANI composite between two WEP layers to create multilayer coatings on mild steel substrates.
Among all of the Ti 3 C 2 T x /PANI composite coatings, Ti 3 C 2 T x /PANI with a 1:2 ratio exhibited the best corrosion protection (Figure 6c). The initial impedance modulus after 1day immersion in 3.5% NaCl was 7 × 10 8 Ω·cm 2 and decreased to 1 order of magnitude to 1.05 × 10 7 Ω·cm 2 after 5 weeks of immersion. Furthermore, Ti 3 C 2 T x /PANI (1:2) sprayed with salts for 45 days demonstrated minimum contents of oxygen and chlorine. The excellent corrosion protection was attributed to the 3D structures of Ti 3 C 2 T x /PANI composites, which served as a reservoir and as a trap for corrosive ions.

Ti 3 C 2 T x /Graphene Hybrid Composites.
Graphene has excellent impermeable and physical barrier properties. However, its ability to be dispersed in the polymer matrix is  poor. To improve graphene dispersion, the covalent functionalization of graphene with polymeric materials is an option. Another novel strategy is to combine graphene and Ti 3 C 2 T x to achieve graphene/Ti 3 C 2 T x heterostructures. Since Ti 3 C 2 T x is hydrophilic, the resulting graphene/Ti 3 C 2 T x heterostructures are also hydrophilic and may show beneficial synergistic effects for anticorrosion. Recently, Yan et al., 52 have demonstrated the anticorrosion properties of Ti 3 C 2 T x /graphene heterostructures in which Ti 3 C 2 T x is wrapped by graphene. First, they prepared the Ti 3 C 2 T x /graphene heterostructures by making a graphenewrapped Ti 3 C 2 T x via polydopamine interfacial chemistry (Figure 7a). Ti 3 C 2 T x /graphene was then mixed with the epoxy to make Ti 3 C 2 T x /graphene−epoxy coating composites. The corrosion properties of the Ti 3 C 2 T x /graphene−epoxy coating composites in 3.5% NaCl were studied.

CONCLUSIONS AND FUTURE PERSPECTIVES
We have discussed the current progress on the Ti 3 C 2 T x MXene for anticorrosion applications. Both pristine and functionalized Ti 3 C 2 T x may be used for making anticorrosive coatings. Physical mixing of Ti 3 C 2 T x with waterborne epoxy (WEP) or polyurethane (PU) is the most popular route for preparing anticorrosion coatings. In general, studies on the behavior of the Ti 3 C 2 T x MXene in corrosive environments indicate that its hydrophilicity has both advantages and disadvantages. Ti 3 C 2 T x hydrophilicity improves its dispersibility in WEP and PU matrices. However, the abundant oxygen functional groups on the Ti 3 C 2 T x surface may trigger more corrosion. Ti 3 C 2 T x MXene is prone to chemical degradation due to active hydrolysis and surface oxidation, leading to the formation of titanium oxide, 24 which may impede the longterm corrosion protection.
There are at least three main approaches to slow down or prevent the oxidation and chemical degradation of MXenes and to improve their shelf life. The first route is to improve the purity and stoichiometry of the precursor MAX phase to enhance the quality of the resulting MXene flakes by lowering the number of defects in the resulting MXene 2D flakes. 29 The second approach is to increase the flake size and decrease the concentrations of the defects during the selective etching and delamination processes. 53 The third method is to improve MXenes' oxidation resistance by decorating their surface with organic/inorganic moieties. 31,33,54 The presence of multiple species of ions in-between the MXene sheets can also influence the rate of oxidation.
Surface functionalization of Ti 3 C 2 T x is important to maintain its chemical stability for long-term application in anticorrosion coatings. 36,41 The attachment of organic functional groups was used to functionalize Ti 3 C 2 T x to maintain its chemical stability. 35 A strategy that may further improve the chemical stability of Ti 3 C 2 T x is the covalent functionalization with polymer brushes. 55 However, the grafting of polymer brushes on Ti 3 C 2 T x for its application in anticorrosion is not explored yet. This can be an easier task in comparison to the assembly of MXene with other 2D materials, such as graphene or hexagonal boron nitride. The abundant functional groups on the surface of Ti 3 C 2 T x can be functionalized with initiators to perform surface-initiated atom transfer radical polymerization. Furthermore, direct photografting, known as self-initiated photografting and photopolymerization (SIPGP), can also be used, which is a one-step polymerization process where no initiator attachment is needed. Polymer brush grafting methods via SIPGP have been utilized on other 2D materials such as graphene, 56,57 graphitic carbon nitride, 58 and hexagonal boron nitride. 59 Unlike graphene, 60 Ti 3 C 2 T x -based coatings for anticorrosion are not widely explored and the research area of MXenes for anticorrosion coatings is at its initial stage. Ti 3 C 2 T x is only one composition in the large compositional space of MXenes, which suggests the potential of this field due to the tailorable compositions and properties of MXenes.

BEYOND TI 3 C 2 T X MXENE
Research on the anticorrosive behavior of this large family of 2D layered carbides/nitrides and carbonitrides is very limited and is yet to expand beyond Ti 3 C 2 T x MXene. MXenes consisting of transition metals with high electronegativities may be more effective for inhibiting the propagation of corrosion. The introduction of two different MXene compositions in aqueous or other solvent media together can improve their combined efficacy for inhibiting corrosion. For example, the use of different MXene types mixed together has shown improved electrochemical activity. 61 Preparation of MXenes in nonaqueous, polar solvents can further eliminate the potential of oxidation due to hydrolysis. In addition, a more in-depth understanding of surface terminations' effect toward high impedance behavior and stability in saline environments must be developed, which leads to lower protection/inhibition of corrosion.
Since corrosion is a surface phenomenon, tailoring of surface functional groups is vital to develop coatings that are resistant to oxide growth, therefore requiring more oxidation-resistant species on the basal (outer) planes of MXenes. MXenes with two transition metals, known as double transition metal MXenes, as either random solid solutions or in-plane ordered or out-of-plane ordered MXenes 62−64 can potentially inhibit corrosion via the contribution from the transition metals order/disorder, as well as via providing better control on their charge transport properties. 65 Extending the concept of multiple M elements in MXenes, the newest addition of entropy-stabilized MAX phases and the isolation of high-entropy MXenes with multiple principal metals have further expanded the scope of MXenes for exploration toward anticorrosion materials. 66−69 These systems, where the metal elements have random occupancies, exhibit a high rate of disorder with diverse electronegativities. This can potentially contribute to lower rates of oxidation and extend the service life of coatings due to the availability of nonhomogeneous active sites for corrosion initiation. The diversity of MXenes in terms of constituent elements, layer thicknesses with two to five layers of transition metals, and different surface terminations 70 provide a platform to further expand the available tools in corrosion science and engineering.